My friend, you test me, as you are unequivocally more qualified on this matter, but supposing that this is more a riddle than a question, the “best” submission from the mount is the one your opponent gives you. I would also contend that any submission that does not compromise your position in the attempt is superior to one that does. This being said, a standard arm bar is less ideal than a belly down arm bar or choke.
My personal favorite is the triangle choke. By allowing an opening I encourage an attempt to escape out the back. This affords me several options including the opportunity to maintain a dominant position if I should miss the attempt. Here is an example of the fine gentlemen at Submissions 101 demonstrating the technique.
Bear in mind, this video shows falling to the back to complete the submission which is not necessary and compromises your position. I suggest finishing it from the top.
Now that I have made my case, I would like to know your thoughts.
